The objectives of this study are a) to determine the relative importance of maternal circulating precursor availability and placental biosynthetic capacity in regulating estrogen synthesis in vivo; b) to devise a method for estimation of baboon urinary estrone and describe the pattern of excretion of this estrogen during normal gestation. This is part of a continuing study to evaluate the baboon as a model for human pregnancy. These studies require double radioisotopic tracer techniques to determine the extent of conversion of circulating precursors to estrogens by baboons (P. papio) at mid and late gestation. Existing radioimmunoassay methods will be adopted to permit estimation of total estrone in baboon urine.